The invention relates to a method and an apparatus for automated application of paint film to bodywork parts and to a paint film composite which is suitable for automation. In this case, the invention is based on a method for automated application of self-adhesive film to bodywork parts. German reference DE 196 42 831 A1 and DE 198 09 515 A1 are examples of prior art methods of such application to body parts. With regard to the paint film composite, reference should be made as the known generic type to DE 195 32 998 A1.
In the case of vehicles, the bodywork surface which is located between two adjacent glass surfaces is frequently intended, for stylistic reasons, to have a high-gloss paint film, which is colored black irrespective of the color of the rest of the vehicle, bonded over it, in order to give the impression of a continuous glass surface at this point. To be precise, this relates in particular to the two adjacent vertical frame struts of the window frames in the area of the center pillar of the vehicle. At least one of these frame struts is arranged on the window frame of a side door. Since these bodywork surfaces are generally only in the form of narrow strips, the blank of the paint film to be applied also has a correspondingly elongated form and is not particularly large, so that the corresponding paint film blank can be handled manually by just one person without any problems. Since, on the other hand, the paint film remains on the vehicle throughout its entire useful life, the paint film must be appropriately and permanently adhesively bonded to the bodywork. In this case, it should be remembered that the side struts to be adhesively bonded over in this way are located in the direct field of view of the vehicle users, at least on entering the vehicle, and the requirements for perfect film application are thus particularly stringent.
German reference DE 195 32 998 A1 discloses a flat blank for a self-adhesive paint film, which is intended to be applied without any bubbles or creases to the frame strut of a window frame. In order to avoid trapped air enclosures (bubbles) between the bodywork and the paint film, the latter is provided, at least in areas which are at risk, over the entire surface area with a large number of microscopic venting openings which are arranged in the form of a grid. Any air enclosures can escape without any problems through these venting openings. Because the venting openings are so small they do not interfere—at least initially—with the external appearance of the bodywork part to which the paint film has been applied. The venting openings arranged in the form of a grid are also intended to increase the spatial adaptation capability of the paint film, and to reduce the risk of creases being formed. This document admittedly says nothing about the way in which the film is applied—manually or in an automated form, but the overall impression of this document leads to the supposition that the paint film can be applied only manually. Furthermore, this document does not disclose the complete structure of the film composite, which includes the paint film blank ready for processing, in the delivery state. In general, it should be noted as a critical factor of this prior art that the large number of microscopic venting openings will in fact become visible over the course of time owing to weather influences and aging processes, so that the paint film will become unattractive.
It can be assumed that the paint film blank which is provided with a highly intensively adhesive adhesion bonding layer on its lower face represents a film composite having a total of three layers in the delivery state in which it is ready for processing, in which a protective film or a protective paper is in each case adhesively bonded both over the lower face and on the high-gloss visible face of the paint film blank. In this case, the protective paper on the lower face is provided on the contact face with the adhesive bonding layer of the paint film blank with an antistick coating, so that it can be detached from the intensively adhesive bonding layer of the paint film relatively easily, and in particular without tearing into the paint film. The protective paper on the upper face is provided on the contact face with the outside of the paint film blank with an only slightly adhesive bonding layer, so that this protective paper can also easily be detached from the paint film.
When paint films are being applied manually, the protective film strips on the lower face or adhesive face are removed completely, so that the adhesive face of the paint film is exposed. The protective film strip which still adheres to the visible face of the paint film is set back with respect to the end of the paint film according to the description in the abovementioned German reference DE 195 32 998 A1, so that the remaining film composite has to be handled on the paint film blank itself. For application, the paint film—guided manually at both of its ends—is aligned by eye in the stretched state a short distance above the bodywork part to be bonded over, and one end of the paint film is pressed against the bodywork, in the correct position. One important factor in this case is that the boundary edges of the paint film blank are not covered by any outer protective paper in the end area. While manually maintaining the tensile stress in the paint film which has been firmly adhesively bonded at one end, it is pressed against the bodywork from the end which has been firmly adhesively bonded towards the other end along a progressing pressure line using the hand which has become free. This contact pressure is repeated with an increased linear force by means of a flexible wiper, preferably in the form of a hard piece of felt, in order to improve the adhesion. Once the outer protective strip has been pulled off, those edges of the paint film which project beyond the frame strut at the side are then bent around the edges of the bodywork part, and are likewise pressed on the rear face.
Manual application of paint films to the vehicle bodywork has various disadvantages. On the one hand, the paint films can be positioned with only little accuracy, leading to frequent reworking. In this case, it should be remembered that there is no exact reference mark on the bodywork for alignment of the film blank by eye. If the alignment is not correct, the paint film may either be seated at a slight angle, so that the edges, whose dimensions are short, run poorly, or the film blank is seated too high or too low. When applied to the two window frame struts which are immediately adjacent to one another in the area of the center pillar, a different height position of the paint films may in some circumstances be highly disturbing. A further disadvantage is that, even if the paint film runs smoothly on the bodywork and a large contact pressure is applied to the paint film to press it onto the frame strut, air enclosures frequently occur between the two items, although these are frequently not evident until after a certain delay. The air which is adsorbed on the adhesive layer is initially included in the form of microscopic and inconspicuous small bubbles. This primary air slowly migrates within the adhesive layer and gathers to form larger small bubbles, which produce local bulges in the paint film. These small bubbles which appear only retrospectively must be rectified by reworking, that is to say by deliberately puncturing them with needles. It has also been found that the complete removal of the lower protective film strip, that is to say the exposure of the adhesive face of the paint film, leads to dust particles accumulating on this adhesive layer which, in some circumstances, may be evident in an unpleasant manner on the visible face of the applied paint film. The process of pulling off the protective film strip creates a certain electrostatic potential, which attracts dust particles and bonds them to the adhesive layer. Particles deposited between the paint film and the bodywork can be rectified only by tediously removing the applied paint film during a reworking step and, in some circumstances, applying a new paint film after the bodywork has been cleaned. The reworking (which has many causes) in the case of manual application of paint film has an adverse effect on productivity, and considerably increases the production costs. In addition to all that has been stated, it should be noted that manual application of paint film is monotonous but nevertheless requires a high degree of care, and that powerfully manually pressing the paint film in place is not only very tiring but, if such stress is long-lasting, can also lead to adverse health effects, for example to inflammation of tendons and their sheaths, or to joint pains.
As a result of all these difficulties, efforts are also already being made to change to providing the stylistic function of the paint film, specifically on the central frame struts of the side windows, to give the impression of a continuous glass surface by means of a hard plastic part covered with a high gloss black-colored paint, which is clipped and/or screwed to the frame strut at the appropriate point. Apart from the fact that a hard plastic part such as this, including the fitting means, is considerably more expensive than a paint film blank, this also restricts the internal area by the physical height of the hard plastic part, for predetermined external vehicle dimensions. Apart from this, a hard plastic part such as this and the fitting means are also heavier than a paint film blank.
German reference DE 196 42 831 A1 discloses an apparatus and a method for application during series production of self-adhesive protective film to vehicle bodyworks. The protective film is used as transport packaging for brand-new vehicles while the vehicle is being transported from the factory to the vehicle dealer. The protective film is pulled off the bodywork again before the vehicle is handed over to the customer. Since the protective film is used only temporarily, its adhesion capability is only relatively low. It should be possible to pull it off the bodywork again easily after use, in particular without tearing into it or leaving behind any residues on the bodywork. The film need adhere only sufficiently firmly to ensure that it cannot become detached from the bodywork on its own as a result of the wind of motion and/or weather influences during transportation. The bodywork parts to be protected include, in particular, the large-area horizontal surfaces, that is to say the roof, the engine hood and the trunk lid. The driver's door is also bonded over, for protection reasons.
According to German reference DE 196 42 831 A1, the protective film is provided as a virtually endless material web on a supply roll for application, with the adhesive face of the outer layer within the film coil lying directly on the outer face of the film layer adjacent to it on the inside, that is to say being bonded to it—corresponding to the low adhesion capability of the protective film. A reasonable, rectangular piece is pulled off the supply roll and is cut to length for application of the protective film, with two opposite longitudinal sides of the film blank each being held firmly by a robot-controlled holding strip in the form of a respective suction strip, holding the film blank in the stretched-out state. If required, specific parts, for example for fittings, can be cut out of the film blank in this stretched-out state. The film blank is then aligned above the bodywork part over which it is to be bonded, by means of the handling robot, and is applied to the appropriate bodywork part with the film blank being moved towards the vehicle surface. Since the bodywork parts to be bonded over are slightly curved, a relatively small contact zone, located approximately centrally, results first of all as the film blank is moved closer to the bodywork component and its size becomes larger as the film blank is moved closer until, finally, the entire horizontal surface is bonded over virtually without any bubbles or creases. The already applied protective film is then pushed on along a progressive pressure line, by means of a weighted felt wiper which corresponds to the shape.
The known application technique for the only slightly adhesive, large-area protective films, which are provided in the form of wound-up, virtually endless, single-layer material webs, cannot be transferred to the application of small-area paint film blanks which are provided in an individually prefabricated form, in each case as a three-layer film composite. Apart from this, the requirements for position accuracy, freedom from bubbles and creases in the applied paint film are considerably more stringent than in the case of protective films.
An object of the invention is to improve the method which forms the generic type and the corresponding apparatus such that paint film blanks can be applied to sought-out bodywork areas in an automated form, such that reworking can very largely be avoided, despite the quality requirements being much more stringent than those for protective films. Furthermore, the invention is intended to provide a paint film composite which is suitable for automation and by means of which the application of the paint film can be carried out in an automated manner.
According to the present invention, a paint film composite, which is suitable for automation, is first of all created and is adhesively bonded onto the bodywork part in an accurate position, and without any bubbles or creases, by means of a robot-controlled application tool that is suitable for this purpose. In order to securely hold the film composite which has been picked up from a table or the like, the two suction grippers which grip the ends are angled in mirror-image form, in order in this way to prevent the ends from becoming detached from the suction grippers. The lower protective strip, which is gripped by a moving gripping tool on a pulling-off lug which projects at the end, is at least partially pulled off, thus exposing the adhesive face of the useful part of the paint film. The film composite, which is held stretched out, is aligned in the correct orientation at a short distance away from the bodywork part which is to be bonded to and is held inflexibly, and the useful part of the paint film is wiped onto the bodywork surface by a moving wiper, from its separated position. The outer protective strip, which is still held firmly by the suction gripper, is then pulled off the applied paint film. Expedient refinements of the invention can be found in the respective dependent claims.